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Journal of the Advanced Undergraduate Physics Laboratory
Investigation
Volume 1 | Issue 1
Article 3
2013
Electrical and Magnetic Properties of High
Temperature Superconductors Using Varying
forms of Data Acquisition
Aryn M. Hays
Austin College, [email protected]
Lindsay Bechtel
Austin College, [email protected]
Colton Turbeville
Austin College, [email protected]
Anthony Johnsen
Austin College, [email protected]
Chris A. Tanner
Austin College, [email protected]
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Opus Citation
Hays, Aryn M.; Bechtel, Lindsay; Turbeville, Colton; Johnsen, Anthony; and Tanner, Chris A. (2013) "Electrical and Magnetic
Properties of High Temperature Superconductors Using Varying forms of Data Acquisition," Journal of the Advanced Undergraduate
Physics Laboratory Investigation: Vol. 1: Iss. 1, Article 3.
Available at: http://opus.ipfw.edu/jaupli/vol1/iss1/3
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Hays et al.: Electrical and Magnetic Properties of High Temperature Superconductors
Introduction
Physicist Heike Kamerlingh Onnes discovered superconductivity in 1911
in the element mercury.1 Superconductivity is described as zero electrical
resistance below a characteristic critical temperature. An electric current flowing
through a loop of superconducting wire can persist indefinitely with no power
source.
Superconductors also have interesting magnetic properties. The Meissner
effect, discovered in 1933 by Meissner and Ochsenfeld2 is the expulsion of a
magnetic field when the superconductor is cooled below the superconductor’s
critical temperature. Once this temperature is reached there is no electrical
resistance, and electrical currents are created on the surface of the superconductor
in order to shield the superconductor from the applied magnetic field. A magnet
can levitate above a superconductor because the surface electrical currents
generate a magnetic field opposing the field of the magnet.
There are two types of superconductors, as shown in Figure 1. Type I
superconductors display the normal phase and the Meissner phase, in which the
magnetic field is completely expelled from the superconductor. Type II
superconductors also display the normal and the Meissner phases. However, at
intermediate temperatures and magnetic fields, they are in a mixed state, in which
the magnetic field penetrates in the material in the form of vortices (normal cores
surrounded by superconducting currents).
Figure 1. Phase diagram of a Type I superconductor (left) and Type II
superconductor (right). For Type I, at temperatures above TC and magnetic fields
above BC the material is normal the normal state, while at temperatures below TC
and magnetic fields below BC the material is normal the Meissner phase. For Type
II, at temperatures above TC and magnetic fields above BC2 the material is in the
normal state. At temperatures below the TC and magnetic fields between BC1 and
BC2 the material is in the Mixed state, while at temperature below TC and
magnetic fields below BC1.the material is in the Meissner phase.
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Journal of the Advanced Undergraduate Physics Laboratory Investigation, Vol. 1 [2013], Iss. 1, Art. 3
Although it was initially thought that the phenomenon of
superconductivity is rare, scientists later learned that it is rather common. As
shown in Figure 2, about two thirds of the known elements display
superconducting properties.3
Figure 2. Periodic table of the elements. The elements with blue background are
superconducting at ambient pressure, the ones with green background are
superconducting under high pressure, while carbon is superconducting only in the
form of carbon nanotubes.
High Temperature Superconductors were discovered in 1986 by Bednorz
and Muller.4 Their critical temperatures are higher than the 77K of liquid
nitrogen. Some of the most studied High Temperature Superconductors are
YBa2Cu3O7, commonly known as YBCO, and Bi2Sr2Ca2Cu3O9, commonly known
as BSCCO.
Superconductors are used in many areas of everyday life. They can be
found in Magnetic Resonance Imaging (MRI), the Large Hadron Collider at
CERN, Magnetically levitated trains (MagLev), energy transmission, transformers,
and motors.5,6
Experimental Method
The superconductors used in this experiment were manufactured by
Colorado Superconducting, Inc. Our superconductor samples have a four-point
probe and a thermocouple attached to them, as seen in Figure 3.
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Hays et al.: Electrical and Magnetic Properties of High Temperature Superconductors
Figure 3. Schematic of the superconductor and the attached wires.7
With this four-point probe we can measure the voltage across the sample
while we send a current through the sample, and also measure the temperature of
our sample with the thermocouple. A thermocouple is a device in which two
wires, made of different metals, are joined at one end, called a junction.7 The
other end is called the reference end. (See Figure 4) The junction is placed in
contact with the superconductor, inside the brass casing. The reference end can
either be held at room temperature (if that temperature is known accurately), or
placed in an ice bath at 0C. When there is a difference in temperature between the
junction and the reference end, a voltage appears across the thermocouple. Our
probe has a type T thermocouple that consists of two different metals, copper and
constantan. As the sample goes between room temperature and liquid nitrogen
temperature (77K), the voltage across the thermocouple changes by less than 7
mV. Due to the small change in voltage, we need a high-resolution voltmeter,
higher than the one of typical hand held digital multimeters (DMMs). We are
employing a Hewlett Packard HP 3478A voltmeter. Published values and tables
provided by the manufacturer of the four-point probe are used to convert the
voltage into temperature.7
REFRENCE END Figure 4. Schematic of a thermocouple.
Magnetic properties - Method A.
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The first method of determining the critical temperature involves
observing the levitating effect after the superconductor has been cooled below its
transition temperature, and recording the temperature at which the magnet stops
levitating, as it warms up and it undergoes the transition from superconducting to
normal.
Electrical properties - Method B.
The sample is first cooled to liquid nitrogen temperature (below its critical
temperature). Subsequently the sample is allowed to warm up back to room
temperature while we monitor the voltage across the sample as a current is sent
through the sample. If we were to connect only two wires to the sample, the
voltmeter would record the sum of the voltages across the sample and the contacts.
The contacts have a resistance much higher than the sample, and the voltage
recorded would be mostly due to the contacts, hence we would not be able to
extract the sample resistance. The four-point probe eliminates this problem.
Figure 5 shows a schematic of the sample, the wires attached, and the contact
resistance due to each of the four wires.
Figure 5. A schematic of the breakdown of the sample and contact resistance.
Four contacts are applied to the sample, yielding R1, R2, R3, and R4 as contact
resistances. The different sections of the sample, between the points were the
contacts were made, have resistances R12, R23, and R34. The voltage wires are
connected to a voltmeter of input resistance Ri.
When a power supply is attached to the system, a current is set up in the
circuit. Most of the current circulates on the path containing R1, R12, R23, R34, and
R4. Due to the high internal resistance Ri of the voltmeter (of the order of MΩ),
the current on the R2, Ri and R3 path is negligible. Therefore, the voltage that the
voltmeter will record is mostly due to the voltage across R23, which is part of our
sample. The resistance can be determined through the relationship described in
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Hays et al.: Electrical and Magnetic Properties of High Temperature Superconductors
equation 1 between voltage (V), resistance (R) and current (I) known as Ohm’s
Law.
Eq. (1)
R (ohms) = V(volts) / I(amperes)
Method B1.
In this method the experimenters use the four-point probe, a BK ToolKit
2704B ammeter to view the current through the sample and a Hewlett Packard HP
3478A voltmeter to record the voltage between points 2 and 3 of the sample. The
experimenters enter the values in the laboratory notebook, and then use data
analysis software to calculate the resistance and plot it as a function of
temperature. With this approach it is hard to record all the values from the
ammeter and two voltmeters (one for the voltage across the sample, the other
across the thermocouple) at the same time, making it difficult to obtain accurate
results.
Method B2.
In this method the experimenters use the four-point probe in conjunction
with a LabVIEW (Laboratory Virtual Instrument Engineering Workbench),
created by National Instruments. LabVIEW is a program is a graphical
programming language that uses icons instead of lines of text to create
applications. LabVIEW programs are called Virtual Instruments, or VIs for short.
Many VIs are already developed by National Instruments programmers, and can
be used by the experiments in their particular application. In our experiment, the
Ammeter and the Voltmeters are replaced by a NI 9219 DAQ (data acquisition)
board, and the wires are connected to the analog input and thermocouple input
pins on the DAQ board. The DAQ transmits the data to the computer and
subsequently data analysis software is used to calculate the resistance and plot it
as a function of temperature. This method is automated and data can be collected
fast and accurately.
A program in LabVIEW has two different windows. One of these
windows is called the front panel. The front panel is what the user sees and also
displays the data while it is being taken. The second window is called the block
diagram. This is where the programmer uses VIs to execute the program. We
developed our own program in order to measure our HTS samples. Figure 6
shows the block diagram, while Figure 7 shows the front panel of the LabVIEW
program we built. With our program we simultaneously measured a YBCO and a
BSCCO sample.
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Figure 6. The block diagram of the program we built to collect the voltage vs.
time data for the two superconductors.
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Hays et al.: Electrical and Magnetic Properties of High Temperature Superconductors
Figure 7. The front panel of the program we built to collect the voltage vs. time
data for the two superconductors.
Experimental Results
Magnetic properties -­‐ Method A As the sample warmed up, we observed a voltage of 5.71 mV across the
thermocouple when the magnet no longer levitated. This corresponds to a
temperature of 95 Kelvin. Our experimental measurement was fairly close to the
accepted value of 92 Kelvin for the critical temperature of YBCO.
Electrical Properties - Method B1
Figure 8 depicts our measurements of electrical resistance versus
temperature. The critical temperature is where YBCO, as it is warming, no longer
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has zero resistance. Our graph shows the transition temperature at approximately
103 Kelvin, higher than the expected 92 Kelvin.
Resistance v. Temperature 8 Resistance (mΩ) 7 6 5 4 3 2 1 0 0 25 50 75 100 125 150 175 200 225 250 Temperature (K) Figure 8. The Resistance vs. Temperature for the YBCO superconductor, as
measured through Method B1.
Method B2.
Figure 9 depicts our measurements of electrical resistance versus
temperature. With this method the computer collected more data points, as it was
able to take data at smaller time intervals. The critical temperatures for the YBCO
and BSCCO samples were determined to be 115 K and 130 K, respectively,
higher than the accepted transition temperatures of 92 K for YBCO and 110 K for
BSCCO.
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Hays et al.: Electrical and Magnetic Properties of High Temperature Superconductors
Figure 9. Resistance vs. Temperature for both YBCO and BSCCO
superconductors.
Discussion
Both methods, B1 and B2, show a transition temperature higher than the
accepted value. This discrepancy can be caused by a few factors: as shown in
Figure 4, T2 represents the temperature of the sample, while T1 is the reference
temperature. Most thermocouple data considers T1 to be room temperature, and
an inaccurate value of the latter can produce an offset in the data. For Method B1,
we placed the reference junction, T1, in an ice bath, and used the thermocouple
tables with 0° C reference. In Method B2, the NI 9219 DAQ board has a cold
junction compensation (CJC) that was supposed yield more accurate temperature
measurements. Given that we obtained more accurate results with Method B1
and the ice bath, we conclude that the CJC did not perform as expected.
Even with the ice bath, our results for the transition temperature are still
not consistent with the accepted values. This discrepancy could also be attributed
to artifacts in our measurements: the thermocouple measures the temperature on
the surface of the superconductor. We took our measurements while warming the
sample up. The surface warms up faster than the middle of the superconductor,
hence it is possible that we are recording higher values for the temperature, and
hence, for the transition temperature.
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Future improvements would involve more accurately determining the
temperature with the thermocouple. This can be accomplished by recording the
temperature of ice water and liquid nitrogen, and seeing if there are any offsets
from the accepted values. These offsets could then be subtracted from the
recorded data. Devising a method to more slowly warming up the superconductor
may result in less temperature difference between the surface and the middle of
the superconductor, and yield better data.
Conclusion
We characterized HTS of YBCO and BSCCO through magnetic and
electrical measurements. We were able to make our measurement process
automatically recorded by the computer by writing a LabVIEW program. We
observed the absence of electrical resistance and the Meissner effect while in the
superconducting state. We also observed and characterized the transition from the
superconducting to the normal state. Our results for the transition temperature are
close to the accepted values. Future improvements involve a more accurate
temperature measurement, possibly with an ice bath in conjunction with the DAQ
board.
References
1 H. K. Onnes (1911) Commun. Phys. Lab.12,120, 2 Meissner, W.; R. Ochsenfeld (1933) Naturwissenschaften 21 (44): 787–788. 3 (2007, May). Type I Superconductors. Retrieved from
superconductors.org/type1.htm
4 J. G. Bednorz and K. A. Müller (1986). Z. Physik, B 64 (1): 189–193 5 Butch, N.P., de Andrade, M.C., Maple, M.B., (2008) American Journal of
Physics 76, 106 6 Preuss, P. (2010) Lawrence Berkeley National Laboratory News
http://newscenter.lbl.gov/feature-stories/2010/09/10/superconductors-future/
7 Colorado Superconducting, Inc. (2001) Retrieved from
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